Hydraulic power transfer device



p 1970 c. o. WEISENBACH 3,528,

HYDRAULIC POWER TRANSFER DEVICE Filed Jan. 17, 1969 ENGINE Io T" I UTILIZATION I20 UTIUZATION M50 cIRcUIT FIGJ TORQUE FLOW FIG. 3 Q

I INVENTOR I CHARLES o. WEISENBACH 2 DISCHARGE g PRESSURE BY "9/ i ATTORNEYS United States Patent F 3,528,244 HYDRAULIC POWER TRANSFER DEVICE Charles 0. Weisenbach, Watertown, N.Y., assignor to General Signal Corporation, a corporation of New York Filed Jan. 17, 1969, Ser. No. 791,890 Int. Cl. F01b 23/08 U.S. CI. 60-53 Claims ABSTRACT OF THE DISCLOSURE A device for transferring hydraulic power back and forth between separate hydraulic systems without intermixing the oils used in those systems. The power transfer device comprises a pair of motor-pump units whose drive shafts are interconnected, and control means which insures that the driven or pumping unit operates at a smaller displacement per unit of time than the driving or motoring unit. In the illustrated transfer device, the units run at the same speed, and the control means responds to the difference between the pressures at their high pressure ports and sets the motoring unit to a maximum displacement per revolution and the pumping unit to a minimum displacement per revolution.

BACKGROUND AND SUMMARY OF THE INVENTION A relatively new product in the aircraft hydraulic field is a device for transferring hydraulic power between two separate systems without intermixing the oils in those systems. In essence, the power transfer device comprises a pair of identical, fixed displacement hydraulic motorpump units whose drive shafts are interconnected so that each can drive the other, and one of which is connected hydraulically into each system. Depending upon which system is at the higher pressure, the unit associated with that system will operate as a motor and drive the other as a pump. In this way, the transfer device acts to supplement the normal supply pump in the system on which the greater flow demand is imposed, and thereby obviates use of unusually large supply pumps.

The conventional power transfer device is relatively simple, but has the disadvantage that the output pressure of the pumping unit is inherently considerably lower than the input pressure of the motoring unit. This characteris tie is attributable to the inefiiciencies of the units. The torque required to drive the pumping unit varies inversely with its torque efficiency, whereas the output torque of the motoring unit is a direct function of its torque efficiency. Therefore, since the displacements per unit of time are equal, the pump output pressure equals the product of the two efficiencies and the motor inlet pressure. In a typical case where each unit has a torque efficiency of 90%, the maximum output pressure which can be produced by a motor inlet pressure of 3,000 p.s.i. is 2,430 p.s.i. (i.e., .90 .90 3,000). This, of course, means that the transfer unit would remain stalled, and thus transfer no hydraulic power, until the pressure in the system subjected to the higher flow demand decreases about 600 p.s.i. below the pressure in the other system. Obviously, this performance is far from ideal.

The object of this invention is to provide an improved power transfer device which is effective to transfer hydraulic power at much smaller differentials between the pressures in the two systems. According to the invention, the transfer device is provided with control means which acts automatically to insure that the unit which is pumping has a smaller displacement per unit time than the unit which is motoring. Preferably, the units are identical and run at the same speed. The desired difference between the 3,528,244 Patented Sept. 15, 1970 displacements per unit of time is achieved by a control means which responds to the difference between the pressures at the high pressure ports of the units and adjusts their displacements per revolution so that the motoring unit always operates at a maximum displacement and the pumping unit operates at a minimum displacement. The ratio of the minimum displacement to the maximum displacement is set to a value just slightly greater than the product of the torque efficiencies of the two hydraulic units. This scheme reduces the torque required to drive the pumping unit by an amount which substantially off sets the inefficiencies of the motor-pump units and thus enables the device to transfer hydraulic power at a higher ratio of pump discharge pressure to motor inlet pressure than has heretofore been possible.

BRIEF DESCRIPTION OF THE DRAWING The preferred embodiment of the invention is described herein with reference to the accompanying drawing in which:

FIG. 1 is a schematic diagram showing the improved power transfer device in a typical aircraft environment.

FIGS. 2 and 3 are graphs illustrating some of the performance characteristics of the hydraulic units.

DESCRIPTION OF PREFERRED EMBODIMENT As shown in FIG. 1, the improved power transfer device 10 is used to transfer hydraulic power in opposite directions between a pair of separate hydraulic systems 11 and 11a. Each system includes its own reservoir or tank 12 or 12a, a pressure compensated, variable delivery supply pump 13 or 13a which is driven by one of the aircrafts engines 14 and 14a, and a fluid utilization circuit 15 or 15a which usually includes a plurality of closed center distributing valves and several work-performing hydraulic motors or cylinders. The work cycles of the two systems are not identical, so sometimes supply pump 13 is subjected to the larger flow demand and system 11 operates at the lower pressure, and other times the larger demand is imposed on pump 13a and system 11a operates at the lower pressure. Thus, the device 10 must be capable of transferring power in either direction, i.e., it must transfer power to whichever system is subjected to the higher flow demand.

The power transfer device 10 comprises a pair of identical motor-pump units 16 and 16a whose drive shafts 17 and 17a are directly interconnected so that either unit can drive the other and both run at the same speed. The high pressure ports 18 and 18a of the two units are connected, respectively, with the discharge conduits of supply pumps 13 and 13a, and the low pressure ports 19 and 19a communicate with the separate reservoirs 12 and 12a, respectively. Units 16 and 16a may be of the rotary cylinder barrel, longitudinally reciprocating piston type and preferably they are embodied in a single package and utilize a common drive shaft. Of course, since intermixing of the oils in the two systems 11 and 11a cannot be tolerated, this package must include appropriate seals to prevent cross leakage.

The displacement per revolution of each motor-pump unit can be varied between the same minimum and maximum limits by a control element 21 or 21a. These elements are positioned by control means comprising an interconnecting link 22, and a pair of fluid pressure motors 23 and 23a which respond, respectively, to the pressures at high pressure ports 18 and 18a. The arrangement is such that the control element 21 or 21a of the motorpump unit subjected to the higher pressure always is positioned in the maximum position, and the control element of the other unit is positioned in the minimum position. Thus, the displacement per unit of time of the unit which is motoring always is greater than the corresponding displacement of the unit which is pumping.

When device 10 is transferring hydraulic power, units 16 and 16a run at that speed at which the output torque of the motoring unit equals the torque required to drive the pumping unit. Therefore, the relationship between motor inlet pressure and pump discharge pressure may be expressed by the equation P, Q2 N N P where P and P represent pump discharge pressure and motor inlet pressure, respectively (assuming that both reservoirs 12 and 12a are at zero pressure),

Q and Q represent the displacements per revolution of the pumping and motoring units, respectively, and N and N represent the torque efficiencies of the ptunping and motoring units, respectively.

The ratio of the minimum to the maximum setting of each of the control elements 21 and 21a is set to be just slightly greater than the product of N and N This olfsets almost completely the effect of unit inefficiency and allows transfer device I10 to be effective at pump discharge pressures which closely approach motor inlet pressure.

The operation of transfer device 10 can best be described with reference to the graphs of FIGS. 2 and 3. In FIG. 2, the curve M represents the relationship, at maximum inlet pressure, between output torque and speed for the motor-pump unit 16 or 16a which is motoring. The family of curves P P etc., represents the torques required to drive each unit as a pump at different levels of discharge pressure; the curve P denoting the torque at a high pressure level just slightly below the maximum (i.e., just below the full cut-off setting of the compensators used in supply pumps 13 and 13a), and the curves P to P denoting the torques for progressively lower discharge pressures. The output torque of the motoring unit decreases with speed because of the increased viscous drag on the rotating parts and the increased pressure drop through the internal fluid-conveying passages. For the same reasons, the torque required to drive the pumping unit increases with speed.

When the pressures in systems 11 and 11a are equal and a maximum, transfer device 10 will be stalled, i.e., units 16 and 16a. will be at rest, and it will transfer no hydraulic power. However, as the flow demand imposed on system 11a increases and its pressure decreases relatively to the pressure in system 11, control motors 23 and 23a will shift elements 21 and 21a to their illustrated positions, if they are not already in those positions, and unit 16 will become a motor and commence to drive unit 16 as a pump. Therefore, unit 16a will deliver oil under pressure to utilization circuit 15a and thereby supplement supply pump 13a. The discharge rate of unit 16a depends upon the rotary speed of device 10 and this, in turn, de-. pends upon the prevailing pressure in system 1111. If the pressure is at a relatively high level, then, as shown in FIG. 2, transfer device 10 will run at the low speed S corresponding to the point of intersection of curves P and M. At this speed, flow curve Q shows that the delivery rate of unit 16a will be at the low Q level. On the other hand, if the flow demand imposed on system 11a is such that the pressure drops to a lower level, the torque required to drive unit 16a will decrease as shown by the family of curves P P and the units 16 and 16a will accelerate to a higher speed, such as S S S S or S This, of course, causes the discharge rate of unit 16a to increase as shown by the curve of FIG. 3.

As the flow demand imposed on pump 13a decreases and the pressure in system 11a rises, the torque required to drive unit 16a will increase. As a result, the speed of both motor-pump units and the discharge rate of unit 16a will decrease. When the pressure in system 11a rises slightly above the P level, and thus closely approaches the pressure in system 11, the units will stall, and power transfer will cease.

In cases where pump 13 is subjected to the larger flow demand, the pressure in system 11 will decrease below the pressure in system 11a, and motors 23a and 23 will shift control elements 21a and 21 to the maximum and minimum displacement positions, respectively. Under this condition, unit 16a acts as a motor and drives unit 16 as a pump. Therefore, unit 16 supplements supply pump 13 in the same Way as unit 16a supplemented pump 13a in the opposite mode of operation. Of course, in this case curve M of FIG. 2 represents the torque-versus-speed relationship for unit :1-6a at maximum pressure, and the curves P -P and Q apply to unit 16.

Although FIG. 2 shows only the limiting torque curve M for the motoring unit, it will be understood that there is a family of such curves similar to the group P to P and that power transfer will occur in either direction even though the inlet pressure to the motoring unit is below the maximum (assuming, of course, that this pressure is still higher than the discharge pressure of the pumping unit).

It also should be noted that since the operating speed of device 10 varies directly with the dilferential between the pressures in the two systems 11 and 11a, over-speeding of the units 16 and 16a may be a problem if widely different system pressures are encountered. In these cases, excessive speed can be avoided by including flow limiting mechanism which keeps to an acceptable value the rate at which oil is delivered to the units 16 and 16a when each is motoring.

Although, in the illustrated embodiment, the required difference between the displacements per unit of time of the two motor-pump units is achieved by changing their displacements per revolution, it should be understood that the desired end result of the invention can be realized by causing the units to operate at the same displacement per revolution, but at different speeds. This alternative involves interconnecting the drive shafts of the units through a gear train which insures a speed reduction from the motoring unit to the pumping unit regardless of the direction of power transfer.

I claim:

1. A power transfer unit comprising (a) a pair of hydraulic motor-pump units (16 and 16a) interconnected so that each can drive the other and each including high (18 or 18a) and low (19 or 19a) pressure ports; and

(b) automatic control means (21., 21a, 22, 23, 23a) effective when the first unit 16) is driving the second (16a) for operating the second unit at a smaller displacement per unit of time than the first, and, when the second unit (16a) is driving the first unit (16), for operating the first unit at a smaller displacement per unit of time than the second.

2. A power transfer unit as defined in claim 1 in which (a) the hydraulic units (16 and 16a) are of the type affording variable displacement per revolution; and

(b) the control means (21, 21a, 22, 23, 23a) comprises means for varying in reverse senses the displacements per revolution of the two units in accordance with the relative pressures at the high pressure ports 18, 18a), whereby the first unit (16) has the greater displacement per revolution when its high pressure port (18) is at the higher pressure, and the second unit (16a) has the greater displacement per revolution when its high pressure port (18a) is at the higher pressure.

3. A power transfer unit as defined in claim 2 in which References Cited the interconnection between the hydraulic units is such UNITED STATES PATENTS that both units run at the same speed.

4. A power transfer unit as defined in claim 2 in which 2'301098 11/1942 y (a) the two hydraulic units are identical; and 5 3443379 5/1969 Welsenbach' (b) the control means varies the displacements per 3,448,577 6/1969 Crawfori h iiii z i z l between the same EDGAR W. GEOGHEGAN, Primary Examiner 5. A power transfer unit as defined in claim 4 in which the interconnection between the hydraulic units is such 10 6O 97 that both units run at the same speed.

US. Cl. X.R. 

